The invention relates to a combustion method, in particular for a four stroke reciprocating piston internal combustion engine with a combustion chamber, the volume of which can be changed by a reciprocating piston. Fuel can be introduced directly into the combustion chamber. At least one gas exchange inlet valve and one gas exchange outlet valve for a charge cycle is provided. The combustion chamber has a minimal volume at a top dead center of the charge cycle and at the ignition top dead center.
In the case of direct injection internal combustion engines with auto-ignition, the homogeneous lean mixtures of fuel and air often ignite automatically, so that high efficiency and improved exhaust gas emissions are achieved. In such so-called HCCI (homogeneous charge compression ignition) and/or CAI (cold air intake) internal combustion engines, also known as internal combustion engines with chamber ignition combustion, a lean base mixture of air, fuel and retained exhaust gas is usually formed in the lower partial load range and automatically ignited. At higher loads, the auto-ignition also produces a steep rise in the pressure in the combustion chamber, a feature that would have a negative impact on the operation.
DE 198 10 935 C2 discloses, for example, a method for operating an internal combustion engine that operates according to the four stroke principle. In this case, a homogeneous, lean base mixture of air, fuel and retained exhaust gas is formed, and this base mixture is burned following compression ignition.
At the same time, an activation phase is inserted in order to expand the operating range of the engine with compression ignition. During the compression of the retained exhaust gas, a quantity of activation fuel is injected into the combustion chamber and distributed as homogeneously as possible with the other constituents of the mixture in the combustion chamber. In this way thermal energy is introduced into the mixture by way of compression, so that a chemical reaction, that is an ignition in the vicinity of the top dead center of the charge cycle, is introduced. The point of time and the quantity of the activation injection can be used to control the ignition point of the fresh charge during the main combustion process.
In the case of a dynamic operation of the internal combustion engine, that is, in the case of variable rotational speed, it is very difficult with the known combustion method to control with precision the ignition point by means of the point in time and the quantity of the activation injection.
The object of the present invention is to provide a combustion method, in particular for a four stroke reciprocating piston internal combustion engine, with which a very precise control of the ignition point is possible.
This object is achieved with a combustion method which introduces a fresh gas into the combustion chamber in an intake phase; introduces a primary quantity of a fuel into the combustion chamber during the intake phase and/or a compression phase; compresses the fresh gas and fuel in the compression phase; ignites a mixture, which comprises fresh gas and fuel and is formed in the combustion chamber; expands and discharges an exhaust gas, which has formed by the combustion process, in an expansion phase, wherein a pilot quantity of fuel is introduced before the primary quantity of fuel is introduced intermediate products of the pilot quantity of fuel are formed; and the primary quantity of fuel is introduced during the compression phase such that complete ignition of the mixture of the fresh gas and the intermediate products is suppressed, and additional intermediate products are formed, until a controlled ignition of the mixture and the additional intermediate products takes place.
It could be demonstrated numerically that an uncontrolled chain reaction (auto-ignition of the intermediate products of the fuel/air mixture) can be suppressed in a targeted manner by the presence of fuel molecules, thus, delaying the combustion process, and its start can be monitored and controlled. Intermediate products are defined as a fuel that has a higher rate of combustion than the basic fuel. This fuel can be, for example, a mixture comprising fuel, formaldehyde and hydrogen peroxide. One possibility for producing the intermediate products is the so-called cold flame or cold combustion. This chemical process occurs in a temperature range between 700 K and 1,000 K.
At this point, according to the invention, the combustion is delayed for a period of time through the continuous addition of fuel (“subsequent feeding with additional fuel”) until the desired ignition point is reached. Thereafter, the combustion can begin in a controlled manner by external ignition or auto-ignition. Then, this combustion process runs very quickly and stably, and the NOx level remains low on account of the very fast and late reaction. In parallel, the result of the high ignition quality of the mixture and/or the high flame propagation rate is a very lean combustion process, which contributes significantly to decreasing the combustion temperatures. The fact that such a combustion sequence of the invention leads to very low NOx emissions is documented by the results obtained from a lean combustion process with explosive fuels, like hydrogen or reformer gas. In the latter case, for example, H2 enriched gas is admixed to the gasoline. However, in principle the combustion method of the invention can be used for almost any kind of fuel, such as gasoline, diesel, etc. In order to produce intermediate products, it is necessary to use a fuel that is characterized by a distinct low temperature kinetic behavior. This is the case with the higher hydrocarbons, for which reason fuels, such as ethanol or natural gas, are not really suited for the method, according to the invention.
Preferably, the intermediate products are formed by an undercut of the gas exchange valves during the top dead center phase of the charge cycle (LOT phase), since then in the compression phase the intermediate products are present in a very homogenous state in the combustion chamber.
According to further aspects of the invention, the ignition can occur either by way of an external ignition, such as with a spark plug, or by stopping the injection of additional fuel, with subsequent auto-ignition.
According to a further aspect of the invention, it is possible to decrease the emission even more by mixing the fresh gas at least partially with exhaust gas. In this case it concerns, for example, an external exhaust gas recirculation.
In addition to a stoichiometric operation of the internal combustion engine, a lean operation is also possible. To this end, the exhaust gas in the combustion chamber is formed at least partially with a high excess of oxygen.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings explaining two operating modes.